A Guide to... Petrol or Diesel?

A Guide to... Petrol or Diesel?

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Petrol or diesel? It’s a common question when people look to update their car.

Unfortunately, the answer isn’t straightforward. It depends on individual needs – and what sort of impact you want to make on the environment and public health.

Typically, diesel engines make more sense in trucks and in vehicles used for towing, because of the pulling power of the engine at low revs. Diesel engines can also deliver better economy, especially on the open road, so they are often well suited to motorists who do a lot of country driving.

But in most cases diesel-powered cars cost more to buy than the same car with a petrol engine. On a popular European hatchback, for example, the petrol version is $30,000 and the diesel version is $33,000 – a 10 per cent premium.

Will you get that money back in fuel-cost savings? That depends on the price of the fuel and how far you drive each year. The price of unleaded fuel rises and falls more sharply – and more often – because there is more demand. More than three quarters of all vehicles on the road run on unleaded.

Diesel, meanwhile, is mostly bought by big fleet operators and mining and industry contractors, who buy in bulk. In fact, only about 25 per cent of diesel sold in Australia is pumped through retail service stations. Less demand equals less competition, and less price fluctuation. In 2008 diesel was up to 40 cents per litre dearer than petrol, which made diesel a much less attractive option for the average motorist.

However in the first half of 2009 prices of both fuels have generally been on par, which meant diesel drivers were able to drive the same dollars further, putting them well in front. But this period of uncharacteristically cheap diesel was expected to be short-lived because the Global Financial Crisis had reduced demand in the mining and industry sectors, and this led to an oversupply of diesel.

With all of the above in mind, there is yet another factor to consider in the petrol versus diesel debate: emissions. Burning a litre of diesel creates 17 per cent more CO2 than burning a litre of petrol (2.7kg versus 2.3kg). But a litre of diesel can give a 25 to 30 per cent longer driving range than a petrol-powered car. This means that, on balance, diesel engines typically produce fewer greenhouse gas emissions than petrol engines.

Sounds straightforward doesn’t it? Unfortunately we’re not done yet.

In recent times, we’ve been focused on fuel economy and carbon dioxide emissions and patted ourselves on the back for knowing what the numbers mean (in both cases, low numbers are good). But this is only scratching the surface. If we dig a little deeper, there are more harmful emissions we should start to take more notice of.

Both petrol and diesel fuels produce emissions that are harmful to our health, but diesel is a more serious pollutant. Both fuels produce similar amounts of hydrocarbons, toxic air pollutants and carbon monoxide, but diesel produces significantly more oxides of nitrogen (NOx) and particulate matter.

One example: the Mini Cooper diesel has a fuel economy rating of 3.9L/100km, the same as the petrol-powered Toyota Prius hybrid. But the Mini emits 56 times more oxides of nitrogen than does the Mini (0.003 versus 0.168).

The Federal Government’s Green Vehicle Guide says diesels are marked down because “their contribution to air pollution is generally higher than that of comparative petrol or LPG vehicles”. “Of most concern are particulate matter and nitrogen oxides (NOx) which can cause a range of adverse health effects. These emissions are generally higher in diesel vehicles compared with petrol or gas vehicles.”

The NSW Department of Environment and Climate Change says particulate matter can “cause or aggravate: cardiac and respiratory disease, acute bronchitis in adults and children, reduced lung function and asthma attacks”.

It can also cause premature death for people with pre-existing heart and lung conditions. Oxides of nitrogen can restrict lung function and increase the chance of respiratory infections. Basically, unleaded fuel might be worse for the planet, but diesel fuel is worse for our health.

A June 2005 report by the Bureau of Transport and Regional Economics put the annual death toll from vehicle exhaust pollution at between 900 and 2000 people – more than the national road toll. “Diesel exhaust has been linked in numerous scientific studies to cancer, the exacerbation of asthma and other respiratory diseases,” the report says.

This is why the quality of diesel fuel itself has been forced to improve over the past decade and car makers have been forced to introduce particulate filters on diesel vehicles. Even more stringent restrictions are due in 2014.

Oddly, however, the next step in emissions standards, known as “Euro V” and due to come into effect in September 2009, will mean that diesel engines in passenger cars will be allowed to emit 180mg/km, while petrol-powered cars will only be allowed to emit 60mg/km.

To help meet these targets, there is a new generation of so-called “clean” diesel engines that run on a new generation of diesel fuel. The sulphur content of diesel dropped from 500 parts per million to 50ppm in 2006 and was due to fall again to 10ppm in 2009 (to bring Australia into line with European regulations), although this deadline has been extended.

But some experts are already beginning to question the effectiveness of the “cleaner” diesel, the new generation of particulate filters, and the way emissions are measured. Britain's Department for Environment, Food and Rural Affairs says ultra-fine particles may still be dangerous because they can dissolve in the lungs. Further, current vehicle emission and air quality measuring procedures are based on weight rather than surface area.

Ultra-fine particle emissions weigh very little but have a relatively enormous surface area when compared with larger coarse particles. A billion ultra-fine particles can weigh the same as one coarse particle, yet have 1000 times the surface area.

While this debate continues, sales of diesel cars are still growing in Australia. In the first six months of 2009, one in four of all new vehicles was powered by a diesel engine. A decade ago, diesel-powered vehicles accounted for one in 10.

But is this a good thing? Toyota, the world’s biggest car maker, believes petrol-electric hybrid power will ultimately replace diesel power because the emission restrictions on diesel vehicles will be so strict that they may be regulated and priced out of existence.

The theory is that by the time you take into account the cost of urea injection, which has to be filled up by the dealer between service intervals, the soot trap or particulate filter that has to be burned off by the dealers, as well as a NOx reduction catalyst and the cost of direct injection, a petrol-electric hybrid drivetrain would cost about the same or less than a diesel-powered car – and produce fewer emissions.

Furthermore, new technology petrol engines are starting to deliver diesel-like economy but with super-low NOx emissions. Europe’s biggest car maker Volkswagen is investing in small capacity turbocharged petrol engines, for example. Others are due to follow suit.

Other European brands, such as Mercedes-Benz and Peugeot, believe so-called “clean” diesel engines matched with hybrid electric motors are the way to go.

The final word goes to one seasoned motor industry insider: “When you have a fuel-burning car, you are always going to emit some form of poison from the tailpipe. It then becomes a case of choosing your poison”.

About NRMA Insurance NRMA Insurance is a provider of insurance products, including car insurance and home insurance in NSW, ACT & TAS.

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